The union, which recently marked two years of its dispute with GTR over the role of guards, warned there was potential for massive disruption to the “core” area of London, from London Bridge to St Pancras.

RMT general secretary Mick Cash said the new timetable would place massive strains on infrastructure and staffing levels already under pressure and accused GTR of “winging it with potentially disastrous consequences”.

GTR has urged passengers to check their plans before they travel.

Chief executive Charles Horton said: “We don’t want passengers to get caught out and so we strongly advise them to look up the times of their trains as they will find that from 20 May each and every one of them has changed.

“Due to the sheer scale of the changes, we will have to redeploy a large number of trains and crews and services may not run at normal times during the introductory phase, although the impact on peak-time services during the transition will be minimal.”

Image copyrightRMT

Image caption
The RMT union recently marked two years of its dispute with Govia Thameslink

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